In Situ Bubble Nucleation on Hydrophobic Particles

With this mechanism, gas nucleates and bubbles form selectively on hydrophobic particles. The theoretical basis of flotation by gas precipitation or nucleation was proposed in the 1960s and has recently been extended to hydrodynamic cavitation. The gas nu-cleation mechanism has been used to account for particle-bubble collection in dissolved air flotation. An advantage of this mechanism over a conventional particle-bubble contact mechanism is the elimination of a collision stage, a rate-limiting step in fine particle flotation. However, direct adoption of this technique to mineral flotation faces a number of challenges. Clearly, tiny bubbles in the micro and submicron range generated solely by a gas nucleation mechanism are not sufficient to float coarse mineral particles effectively. However, the collision probability of larger bubbles with the particle-tiny bubble aggregates in the quiescent region may increase. The limited number of bubbles that can be generated by gas nucleation from a supersaturated system does not provide sufficient carrying capacity to float large amounts of solids. To improve solid recovery rates, large volume slurry saturation tanks are needed, presenting extra capital and operating costs.

Alternatively, tiny bubbles and cavities can be formed by the reduction of pressure in a fast-flowing fluid, as indicated by Bernoulli's equation:

in which U is the water flow velocity at a point where the pressure is P, and p is the density of liquid. If the liquid flow velocity exceeds a critical value, the pressure in the liquid stream reduces to a value where the liquid pressure falls below its vapour pressure, at which point cavities form which expand to relieve the differential pressure, a phenomenon called hydro-dynamic cavitation. The presence of solids enhances hydrodynamic cavitation due to the increased turbulence and pressure fluctuations around particles in the stream. As in gas-supersaturated systems, cavities would form preferentially on hydrophobic particles relative to energetically unfavourable hydrophilic solid-liquid interfaces. The principal advantage of exploring hydrodynamic cavitation in flotation is that gas supersaturation of slurry is not required and additional air can be introduced into the system for air dispersion. As a result, hydrodynamic cavitation can be readily implemented in mineral flotation systems.

A convenient way of aiding bubble nucleation and cavitation is by aeration in the feed slurry line. The existence of gas nuclei in water has been demonstrated in coagulation, sedimentation and filtration tests using fine coal and silica with a medium particle size of 5 and 1.5 |im, respectively. The size of gas nuclei in natural water was estimated to be 10 |im. When forcing the water through the tip of a cavita-tion tube at a flow velocity above 8-15 m s_1, micro-size bubbles were observed to form. Numerical simulation confirmed that, at this flow velocity, a pressure close to liquid vapour pressure was attained inside the tip of the cavitation tube, suggesting the formation of bubbles by the expansion of the pre-existing gas nuclei and subsequently filled with liquid vapours. Using a light attenuation method, the onset velocity of bubble formation by hydrodynamic cavitation was found to be dependent on the diameter and length of the nozzle, slurry temperature and initial gas content. With gas-supersaturated water, for example, the onset velocity reduced from 15 to 7ms"1. Adding frother into liquid does not affect the onset of bubble formation by cavitation, but it increases the bubble stability. Sebba has reported the formation of stable bubble swarms of approximately 25-50 |im which he called aprons, generated similarly. Adding a small amount of air into the flowing liquid stream enhanced bubble formation at a reduced liquid flow velocity, which provides a direct justification for feed aeration by hydrodynamic cavitation.

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